# std::format_to

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< cpp‎ | utility‎ | format

Defined in header `<format>` |  |
template< class OutputIt, class... Args > OutputIt format_to( OutputIt out, std::format_string<Args...> fmt, Args&&... args ); | (1) | (since C++20)
template< class OutputIt, class... Args > OutputIt format_to( OutputIt out, std::wformat_string<Args...> fmt, Args&&... args ); | (2) | (since C++20)
template< class OutputIt, class... Args > OutputIt format_to( OutputIt out, const std::locale& loc, std::format_string<Args...> fmt, Args&&... args ); | (3) | (since C++20)
template< class OutputIt, class... Args > OutputIt format_to( OutputIt out, const std::locale& loc, std::wformat_string<Args...> fmt, Args&&... args ); | (4) | (since C++20)

Format args according to the format string fmt, and write the result to the output iterator out. If present, loc is used for locale-specific formatting.

Equivalent to:

1) return std::vformat_to(std::move(out), fmt.str, std::make_format_args(args...));
2) return std::vformat_to(std::move(out), fmt.str, std::make_wformat_args(args...));
3) return std::vformat_to(std::move(out), loc, fmt.str, std::make_format_args(args...));
4) return std::vformat_to(std::move(out), loc, fmt.str, std::make_wformat_args(args...));.

Let `CharT` be char for overloads (1,3), wchar_t for overloads (2,4).

These overloads participate in overload resolution only if `OutputIt` satisfies the concept std::output_iterator<const CharT&>.

If any of the following conditions is satisfied, the behavior is undefined:

- `OutputIt` does not model std::output_iterator<const CharT&>.
- std::formatter<Ti, CharT> does not meet the BasicFormatter requirements (as required by std::make_format_args and std::make_wformat_args) for some `Ti` in `Args`.

### Parameters

out | - | iterator to the output buffer
fmt | - | an object that represents the format string. The format string consists of ordinary characters (except { and }), which are copied unchanged to the output, escape sequences {{ and }}, which are replaced with { and } respectively in the output, and replacement fields. Each replacement field has the following format:
`{` arg-id (optional) `}` | (1) |
`{` arg-id (optional) `:` format-spec `}` | (2) |
1) replacement field without a format specification 2) replacement field with a format specification
arg-id | - | specifies the index of the argument in `args` whose value is to be used for formatting; if it is omitted, the arguments are used in order. The arg-id﻿s in a format string must all be present or all be omitted. Mixing manual and automatic indexing is an error.
format-spec | - | the format specification defined by the std::formatter specialization for the corresponding argument. Cannot start with }.
For basic types and standard string types, the format specification is interpreted as standard format specification. For chrono types, the format specification is interpreted as chrono format specification.
For range types, the format specification is interpreted as range format specification. For std::pair and std::tuple, the format specification is interpreted as tuple format specification. For std::thread::id and `std::stacktrace_entry`, see thread id format specification and stacktrace entry format specification. For `std::basic_stacktrace`, no format specifier is allowed. | (since C++23)
For `std::filesystem::path`, see path format specification. | (since C++26)
For other formattable types, the format specification is determined by user-defined `formatter` specializations.
args... | - | arguments to be formatted
loc | - | std::locale used for locale-specific formatting

### Return value

Iterator past the end of the output range.

### Exceptions

Propagates any exception thrown by formatter or iterator operations.

### Notes

As of P2216R3, it is an error if the format string is not a constant expression. std::vformat_to or std::runtime_format(since C++26) can be used in this case.

### Example

Run this code

    #include <format>
    #include <iostream>
    #include <iterator>
    #include <string>

    int main()
    {
        std::string buffer;

        std::format_to
        (
            std::back_inserter(buffer), // < OutputIt
            "Hello, C++{}!\n",          // < fmt
            "20"                        // < arg
        );
        std::cout << buffer;
        buffer.clear();

        std::format_to
        (
            std::back_inserter(buffer), // < OutputIt
            "Hello, {0}::{1}!{2}",      // < fmt
            "std",                      // < arg {0}
            "format_to()",              // < arg {1}
            "\n",                       // < arg {2}
            "extra param(s)..."         // < unused
        );
        std::cout << buffer << std::flush;

        std::wstring wbuffer;
        std::format_to
        (
            std::back_inserter(wbuffer),// < OutputIt
            L"Hello, {2}::{1}!{0}",     // < fmt
            L"\n",                      // < arg {0}
            L"format_to()",             // < arg {1}
            L"std",                     // < arg {2}
            L"...is not..."             // < unused
            L"...an error!"             // < unused
        );
        std::wcout << wbuffer;
    }

Output:

    Hello, C++20!
    Hello, std::format_to()!
    Hello, std::format_to()!

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3539 | C++20 | out could not be a move-only iterator | it can be
P2216R3 | C++20 | throws std::format_error for invalid format string | results in compile-time error instead
P2418R2 | C++20 | objects that are neither const-usable nor copyable (such as generator-like objects) are not formattable | allow formatting these objects
P2508R1 | C++20 | there's no user-visible name for this facility | the name `basic_format_string` is exposed

### See also

format (C++20) | stores formatted representation of the arguments in a new string (function template)
format_to_n (C++20) | writes out formatted representation of its arguments through an output iterator, not exceeding specified size (function template)

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